Aircraft engines have been made quieter as a result of advanced high bypass ratio engines. High bypass ratio engines derive a substantial fraction of their total thrust from bypass air which is propelled around the core of the engine by an engine-driven forwardly mounted fan. This approach results in less engine noise than pure turbojet engines or low bypass ratio engines.
One approach to further reducing engine noise is to increase the amount of mixing between the high velocity gases exiting the engine, and the surrounding freestream air. In that regard, the use of geometric structures known as chevrons may reduce low-frequency noise by increasing the rate at which the engine flow streams mix with the surrounding freestream air in the aft region of the nozzle. However, in some circumstances existing chevron designs may increase the drag of the duct, thereby decreasing engine efficiency.